Steam will automatically download and install the mod for use in-game. Manual Installation Method
For mods from external sites like Spintires ModsLT or Spintires.nl , follow these manual steps: How To Install Mods In Spintires [Full Game Guide 2014] skachat mody v igru spin tires
Installing mods for (originally released in 2014) involves either using the official Steam Workshop or manually placing files into the game's directory. Official Method: Steam Workshop Steam will automatically download and install the mod